The present invention is directed to a process for laser cutting workpieces in which a focal beam is directed at the workpiece being machined and determining when the laser beam pierces the workpiece by a control parameter that is detected and gives the penetration status of the workpiece as soon as the focussed beam has pierced the workpiece. The invention also relates to a device for laser cutting workpieces with a cutting head which sends out a focussed beam, and having a digital control device which has detectors for control parameters giving the penetration status of the workpiece, an evaluation unit connected to the detectors and a control unit connected to the evaluation unit to control the parameters of the cutting process.
In most cutting work using a focussed laser beam, the beam must pierce the workpiece. In these cases, the earliest possible detection of the time of penetration is of the greatest importance. If a cut is to be made, the piercing of the focussed beam of the workpiece marks the time when the relative movement of the focussed beam and the workpiece necessary to make the cut can start. When workpieces are cut by a focussed beam, the cutting process at a given point ends with the piercing of the workpiece and thus reaches that point in time in the process when the workpiece being machined can be exchanged for another workpiece to be machined or when the focal beam can be directed somewhere else on the workpiece to continue machining with corresponding control of the process or tooling machine. Always, early detection of the time when the workpiece is pierced makes it possible to minimize the elapsed time for the cutting operation and thus optimize the whole process. To prevent defective machining of workpieces, it must be guaranteed that the time of piercing will be detected with great certainty. Thus, in the example of making a cut, the relative movement of the focussed beam and the workpiece may only start and continue when the focussed beam has actually pierced the workpiece.
Conventional processes and devices for early detection of the piercing time of a focussed beam on a workpiece and conversion of early detection of the piercing point into process and device control are described in U.S. Pat. No. 6,008,465 and EP-A-0 344 339.
In U.S. Pat. No. 6,008,465, the resistance value of the electrical resistance of the medium or plasma is detected on a laser cutting machine between the cutting head of the machine and the workpiece being machined. During the penetration phase of the laser beam, the electrical resistance of the plasma has a relatively low value. Upon the piercing of the workpiece, there is a sudden rise in the resistance value. This rise in electrical resistance is converted into a control signal for the laser cutting machine and thus produces relative movement of laser cutting head and the workpiece to make a cut in a predetermined path.
IN EP-A-0 344 339, the intensity of the light emitted from the machining site of the laser beam on the workpiece is used as the control parameter for the process and machine controller. This light intensity is abruptly reduced as soon as the laser beam has pierced the workpiece.
It is the object of the present invention to provide a novel method for controlling the laser beam and machine controls for relative movement of the laser cutting head and workpiece.
It is also an object to provide a novel laser cutting machine incorporating improved sensing of the apparatus processing condition for controlling the cutting and relative movement of the workpiece and cutting head.
It has now been found that the foregoing and related objects may be readily attained in a method for laser cutting of workpieces using a laser cutting installation including a workpiece support, a laser beam generator generating a laser beam, a laser cutting head with a nozzle through which the laser beam exits, means for effecting relative movement between a workpiece on the support and the cutting head, and a controller for controlling the laser beam and the movement means in accordance with parameters for the cutting of the workpiece.
The laser beam is focussed on the workpiece on the support to cause the laser beam to penetrate the workpiece, and parameters of the cutting operation in progress are sensed at least two sensors which generate signals indicative of the status of the penetration of the workpiece. The signals from each of the sensors are evaluated by comparing the sensed parameters in the signals with usability criteria to determine the usability of the signals. The signal which compares most favorably with the usability criteria therefor is selected and transmitted to the controller so that the observed parameters of the selected signal modify the parameters for the cutting of the workpiece.
In the absence of a control signal from the evaluation step, the machine proceeds with the cutting in accordance with parameters then operative in the controller. Generally, the selected signal is operative to control the speed of relative movement between the workpiece and the cutting head.
Desirably, one of the sensors detects the resistance between the nozzle and the workpiece, and another sensor detects the intensity of the light generated at the cutting site. The latter sensor includes a photodiode. In one embodiment of the process, a second sensor detects the intensity of the laser beam upstream in the path from the generator to the nozzle.
Preferably, the evaluating step involves comparing multiple parameters in each signal with the usability criteria, and one of those parameters is the gradient in the path of the control signal over time in relation to a threshold value for the relative control signal level.
A laser cutting machine for conducting the process has a support for a workpiece, a cutting head which emits a focussed laser beam that penetrates the workpiece being machined and finally pierces it, and means for effecting relative movement between the workpiece and cutting head. It includes a digital machine control assembly with at least two detectors to detect control parameters relating to the status of penetration of the workpiece by the laser beam and to transmit signals corresponding to the detected control parameters. An evaluation unit receives the signals from the detectors and includes means for producing a control signal. A control unit is connected to the evaluation unit to receive signals therefrom and to control the parameters of the cutting process.
The control assembly is operative before the workpiece is pierced by the focussed laser beam, and at least two control parameters are detected simultaneously and continuously by the detectors. The evaluation unit is adapted to compare the signals received from the detectors with predetermined usability criteria for the signals received from each sensor, and it determines the detector signal which compares most favorably with its set of usability criteria. A control signal based upon the selected signal is transmitted to the control unit.
If all detector signals fail to meet the usability criteria, the evaluating unit emits no control signal.
Preferably, at least two potential control parameters are detected at the same time, and a single potential control parameter is detected simultaneously several times. In both cases, at least two different potential control signals are generated on the basis of detections of control parameters. Each of these control signals is then checked for its usability, i.e., for its reliability in terms of the penetration status of the workpiece, before the workpiece is pierced. If none of the potential control signals checked proves reliable enough, then it reverts to a control signal different from the potential control signals checked for further process and machine control to ensure functionally proper process and machine control. This xe2x80x9chelpxe2x80x9d control signal can also be produced on the basis of a process parameter that is independent of the actual penetration status of the workpiece, but that also allows it to go back to that penetration status. One example of this that can be given is the length of time the workpiece is acted on by the focal beam, as the basis for producing the xe2x80x9chelpxe2x80x9d control signal which may be stored in the controller. In each case, it is guaranteed that the process and machine control is based on a control signal that has great enough reliability in terms of the penetration status of the workpiece being machined and that accordingly allows the process to run and the machine to operate in a way that is functionally safe. This functional safety of the process and the device in the invention is thus ensured in the event of changing machining conditions as well. But the reliability of the individual control parameters or control parameter detections, in terms of the penetration status of the workpiece, can fluctuate depending on the material or the thickness of the material; multiple detection of control parameters, with subsequent production of several potential control signals, and a check of their usability therefore ensures that the actual control signal last used for process and machine control is a control signal that is reliable in terms of the penetration status of the workpiece being machined.
With the intensity of the process light emitted at the machining site of the focal beam and with the electrical resistance of the medium between the cutting head and the workpiece being machined, control parameters are detected which characteristically reflect the penetration status of the workpiece being machined and whose detection is reliable and possible at a relatively low manufacturing expense. For example, at least one photodiode can be used to detect the intensity of the process light emitted at the machining site of the focal beam.
The value of the gradients of the path of the control signal over time in relation to a threshold value for the gradients and/or the value of the relative control signal level (actual control signal level in relation to a maximum value of the control signal level) in relation to a threshold value for the relative control signal level may be used as the usability criterion by which the potential control signals are checked. The last usability criterion is primary, qualifies and can be met. The relativization of the level of the control signal in the invention is used to make the levels of different control signals produced on different bases comparable to one another.